Optical fiber sensing technology is often used in large-scale, long-distance monitoring, such as security monitoring used in oil pipelines, high-voltage power grids, pipelines, communications cable and other infrastructure. A fiber is used as a sensor to acquire a related disturbance signal in real-time, and a location of the disturbance is determined by analyzing characteristics of light produced by the optical fiber. The structure of a single core feedback optical path is, using a single fiber as sensing fiber, has the fiber itself not closed, and only has a feedback device, such as a mirror, at an end of the fiber constituting an interference optical path. In practice, this structure laying is flexible. The characteristic of such monitoring systems that the light carrying the disturbance information is the light which is transmitted to the end of the fiber, and then reflect by feedback device.
The following is positioning technology of a single core feedback positioning system.
As shown in FIGS. 1 and 2, a sensing section for an optical fiber (optical cable) 1 is at a starting point of the optical fiber (optical cable), and a feedback device 2, such as a mirror, is at an end of the sensing section. Incident light retraces via the feedback device 2. For example, when a disturbance occurs at at point D, whose modulation of a light phase is φ(t), and when the light travels through the disturbance point D, the resulting phase modulation would beφ1(t)=φ(t)+φ(t−T)
wherein, T=2neff L/c·L being the distance between the disturbance point D and the feedback device 2, c being the speed of light in vacuum, and neff being the effective refractive index of the optical fiber.
As shown in FIG. 2, we configure an interference optical path.
The interference optical path includes the following parts: a N*M coupler 3, where N and M are integers; a P*Q coupler 4, where P and Q are integers; optical fiber delayer 5 having a delay of, for example, τ; an optical fiber (optical cable) 6; and a feedback device 2. 3a1, 3a2, . . . , 3aN, 3b1, 3b2 are ports of the coupler 3. 3a1, 3a2, . . . , 3aN are co-rotating ports (with a total of N) of the coupler 3. 3b1, 3b2 are two ports in another group co-rotating ports (with a total of M) of the coupler 3. 4a1, 4a2, 4b1 are ports of the coupler 4. 4a1, 4a2 are two ports in a group co-rotating ports (with a total of P) of coupler 4. 4b1 is a port in another group co-rotating ports (with a total of Q) of the coupler 4. The optical fiber 6 is a sensing optical fiber. The feedback device 2 makes the light transmitted along the fiber go back through the fiber 6 and return to the coupler 4. A light source is input through the port 3a1 of coupler 3, and after splitting in coupler 3, is output respectively through the ports 3b1, 3b2 after being split in the coupler 3. The two optical paths are:                I: 3b1→5→4a1→4b1→6→2→6→4b1→4a2→3b2        II: 3b2→4a2→4b1→6→2→6→4b1→4a1→5→3b1        
The two optical paths join at the coupler 3 again and generate interference, and interference signals are output respectively through the ports 3a1, 3a2, 3aN.
In the interference optical path, the light first enters the delayer 5 and then enters fiber cable 6, and phase modulation applied to the light is:φ2(t)=φ(t−τ)+φ(t−τ−T)
A phase difference between the two coherent interference lights is:Δφ=[φ(t)+φ(t−T)]−[(φ(t−τ)+φ(t−τ−T)]
In a spectrum of the phase difference, there is a frequency drop point or “notch point”. The location of the disturbance can be determined based on the notch point. For example, the “Notch point” is shown in FIG. 3, and in this amplitude—frequency diagram obtained by a time-frequency transformation, “O”s mark the notch point. A relationship between the notch point and the disturbance position is:
                    f        null            ⁡              (        k        )              =                  k        2            ·              c                  2          ⁢                      n            eff                    ⁢          L                      ,          ⁢      (                  k        =                              2            ⁢            n                    -          1                    ,              n        ∈        N              )  
wherein, fnull(k) is frequency of a k-order notch point.
We can see from the above relationship that the coherent light must transmit from the endpoint 1 of the sensing optical fiber 6 to the endpoint 2 and then return to sensing optical fiber 6 in order to carry the position “L” message. However, in practice, due to the structural characteristics of the optical fiber and defects in the optical fiber itself and other reasons, scattered light, such as Rayleigh scattered light and the like, occurs in the optical fiber 6.
As shown in FIG. 4, 7 is a scattering point, a backscattered light goes back to interference structure along the optical cable 6, and therefore there are two beams of light:                I: 3b1→5→4a1→4b1→6→7→6→4b1→4a2→3b2        II: 3b2→4a2→4b1→6→7→6→4b1→4a1→5→3b1        
Because of similar spectral characteristics, the optical paths are equal without a disturbance, and therefore join at the coupler 3 again and will generate interference. Obviously, the information carried by the two beams of interference lights is a length L7 between the point 7 and the disturbance point D. 8 is another scattering point, the length information carried by an interference formed by backscattering is a length L8 between the point 8 and the disturbance point D, and apparently, L7≠L8≠L, since these interferences are mixed at the output. Interference light generated by Brillouin scattering or Raman scattering can be filtered out by an optical filter. However, interference light generated by Rayleigh scattering or interference light generated by a contact point of an optical path, cannot be eliminated by an optical filtering method, which will affect the purity of the useful interference signal, and will directly affect the accuracy of the disturbance L position. Generally, the intensity of an interference generated by backscattered light or a contact is significantly less than the intensity of an interference generated by reflected light (effective interference signal), such that it will not have a significant impact on the effective interference signal, and the accuracy of L can meet the actual needs. But after a monitoring circuit reaches a certain length, and as the scattered light affects the entire line, obvious interference signal distortion is observed and the system cannot obtain a valid interference signal normally. The acquired signal contains not only effective interference signal but also contains a spurious interference signal caused by scattered light.
Similarly, a reflection by a contact point of an optical path can also cause the same adverse effect on the interference signal.
The impact of the scattered (reflected) light in the conventional path is not only the obvious restriction in monitoring system, but also when a large scattering (reflecting) point exists, the system cannot properly perform a test of an optical fiber.
In order to cut the impact of the signal, the invention CN 201010508357.2 (FIG. 5) proposed using the phase generated carrier technology to separate the effective interference phase information from the optical output which is mixed with backscattered light, contact point reflected light interference signal to obtain a pure signal having effective disturbance position information, so as to achieve the purpose of eliminating the impact of back scattered light and the like. In this technology, a phase modulator 9 is positioned close to the feedback device 2 located at the end of the sensing optical fiber (optical cable) 6, and a modulation signal is applied to phase modulator 9. Accordingly, the backward transmitting light is scattered (reflected) by a scattering (reflecting) point (such as scattering points 7,8) and does not reach the feedback device 2 at the end of the sensing optical fiber 6, and its signal is not modulated because not passing through the phase modulator 9. The effective light is reflected by the feedback device 2 and reaches the end and its signal is modulated to a sideband of a fundamental frequency or a double frequency of the modulation frequency which the scattered interference signal cannot reach because of the phase modulator 9, and is separated with the scattered light signals. The extraction of the effective light information can be achieved by corresponding signal processing means, so as to avoid the interference of the scattered light. Since the described technique connects the phase modulator 9 in series with the end of the sensing optical fiber 6, the optical path is phase-modulated and an electrical signal is applied to the modulation signal Therefore, the means connected to the end of sensing optical fiber is an active device that requires power. It is difficult to provide power to the end of sensing optical fiber, therefore the application of the method is limited.